I barely noticed that Forsaken Fortress had no exit... mostly because I remember that later in the game, after getting the King of Red Lions, I could leave anytime I wanted. Besides, there was really nowhere else you would want to go at that point in the game, having no bottles or shops open, and not having seen any other islands.
But honestly, I would never support the idea in anything besides the first dungeon, like it was done in WW. I would be very upset if I ran out of items, and had no escape. Or if I couldn't save in the middle of the dungeon... that was one of the things that bothered me about PH's Temple of the Ocean King.
In MM, you could leave the dungeon without having your progress reset, you just couldn't reset time. So basically, you could clear the way to the boss, and then stop by an item shop in order to pick up some potions before returning... if you had enough time, that is. Also, you COULD save obtaining the dungeon's weapon, which usually allowed you to skip ahead to the boss key and and boss door with less trouble than before.
One of the reasons I enjoy Zelda, is because it doesn't have those kind of frustrating design choices. I can save whenever, and if I lose... just find my way back to where I was before. I would hate having to redo an entire dungeon just because I failed to beat the boss. That kind of thing goes against the spirit of Zelda, and would ruin one of my favorite aspects of Zelda.